Legislation in the UK requires children over three years old and under 12 years old or under 135 cm tall to use a booster seat or booster cushion when travelling in the front or rear of a car or van.
Booster seats and cushions typically comprise a rigid polystyrene or plastic moulded base upon which a child can sit to raise the child to a position wherein the existing car seat belt is positioned correctly on the child's body. The base is normally provided with projecting side members extending upwardly and forwardly from the rear of the base on either side of the base around which at least the lap portion of the seat belt may be guided to retain the booster seat in place and to ensure that the lap portion of the seat belt rests across the pelvic region of the child and not across the child's stomach where it may cause injury.
Booster seats and booster cushions are often categorised by their use and are referred to by groups, namely:—                Group 2: for children weighing 15-25 kgs (33-55 lbs) roughly 4 to 6 years;        Group 3: for children weighing 22-36 kgs (48-79 lbs) roughly from 6-11 years.        
Many booster seats and cushions are designed for children between 15 kg and 36 kg (33-79 lbs), covering both Group 2 and Group 3.
Booster cushions comprise a base cushion for the child to sit upon. Booster seats have an additional backrest attached to the base, typically provided with an upper seat belt guide to hold the upper portion of the seat belt in the correct position across the child's shoulder. Some booster seats are designed to be converted into a booster cushion by detaching the backrest.
Booster seats and booster cushions do not have an integral harness to hold the child in place and rely on the vehicle seat's existing three point seat belt or lap belt to restrain the child. The purpose of the booster seat or cushion is to ensure that the seat belt fits correctly such that the lap belt or lap belt portion of the seat belt extends over the pelvic region of the child, not the stomach, and such that the diagonal portion of a three point seat belt rests over the shoulder of the child, not the neck
A problem with existing booster cushions and seats is that, due to the rigid nature of the base of the cushion, they are bulky and awkward to carry and not very comfortable to sit upon. While attempts have been made to develop inflatable booster cushions that can provide greater comfort and be deflated to reduce the size of the cushion when not in use (see GB 2 442 437), none of the previous attempts have been able to provide adequate performance due to the inherent flexibility of the inflatable cushion and the difficulty of adequately guiding the lap belt portion of the seat belt to ensure that the booster cushion is retained correctly on the vehicle seat and that the lap belt remains in the correct position on the child without resting on the child's stomach with the resulting risk if serious injury in a collision.